A plasma torch is a device which uses an electrical discharge generated in a space or gap between two oppositely charged bodies to heat and to ionize a stream of gas flowing through the gap. The negatively charged body is referred to as an electrode, while the positively charged body is referred to as an anode. The stream of ionized gas produced when the electrical discharge passes through the gas in the gap from electrode to anode is referred to as a plasma jet.
The plasma torch technology described above has presently been put to a number of different uses. For example, the plasma torch has been used both as a welding tool and a cutting tool in high speed manufacturing processes. The plasma torch may also be used as a tool for developing heat resistant materials. Further, this plasma torch technology has been utilized in high temperature furnaces for the incineration of dangerous waste materials, such as medical waste.
One disadvantage of plasma torch technology is the extremely high temperature environment in which the electrode must operate. These high temperatures lead to rapid electrode erosion, as the electrode material melts, evaporates, or is otherwise transported away from the electrode. The immediate consequence of electrode erosion is a degradation of the plasma jet generation. A further consequence of rapid electrode erosion is that users of this plasma torch technology must factor in the costs of providing numerous replacement electrodes, as well as the sizeable cost of the down-time required to replace the electrodes.
In an attempt to control electrode wear through increased dissipation of heat from the operative portion of the electrode, it is common to fabricate the electrode using a heat-resistant insert embedded in a water-cooled holder. The insert is conventionally selected from the group of electrically conductive materials having high melting point temperatures, such as tungsten (W), molybdenum (Mo), tantalum (Ta) and carbon (C).
If the working gas used is air or oxygen, however, an oxidation reaction can occur where the working gas contacts the surface of the insert. As the melting temperature of the oxidation product of those insert materials mentioned above is typically lower than the melting temperature of the insert material, this oxidation reaction may promote and accelerate the rate of melting of the electrode. As a consequence, the erosion of the electrode may still occur at a rapid rate, and the life of the electrode may be correspondingly relatively brief.
Consequently, to reduce the rate of electrode erosion it is also known to use an electrode material with an oxidation product which has a melting point temperature which is equal to or higher than the melting point temperature of the electrode material. Such materials include zirconium (Zr) and hafnium (Hf).
Even with these further precautions, the electrode may fail rapidly because of the heat transfer resistance of the holder material and the heat transfer resistance existing at the insert-holder interface. When the amount of heat generated during the creation of the plasma jet and transferred to the insert exceeds the capacity of the holder, commonly manufactured from copper, to carry the heat away from the insert, the material of the holder surrounding the insert may begin to melt. As a consequence, some of the holder material melt may flow towards the insert, and contaminate the insert material. The impurities thus created in the insert material have a tendency to cause melting of the insert to accelerate, increasing the rate of degradation of the electrode and reducing the life of the electrode.
U.S. Pat. No. 3,930,139 to Bykhovsky et al. describes that the life of an electrode formed using a hafnium or hafnium insert and a water-cooled holder may be increased by placing a spacer between the insert and the holder. Specifically, Bykhovsky et al. suggest that the spacer be formed from aluminum or an aluminum alloy with a radial thickness of 0.01 to 0.2 mm. However, use of aluminum in an oxidizing atmosphere can result in an oxidation product of aluminum being formed, which oxidation product may make the spacer unstable and reduce the durability of the electrode.
U.S. Pat. No. 5,097,111 to Severance describes that the life of an electrode in an oxidizing atmosphere may be increased through the introduction of a sleeve between an insert made, for example, of hafnium and a holder made from copper or a copper alloy. Severance discloses that the sleeve has a radial thickness of at least about 0.01 inches. Severance also discloses that the sleeve is formed of a material having a work function which is greater than that of the material of the holder and the insert, for example, silver.
U.S. Pat. No. 3,198,932 to Weatherly describes that the stability of an electrode in a reactive gas atmosphere may be improved by silver-brazing a zirconium insert into a recess in a silver holder. Specifically, Weatherly discloses a method whereby the insert is first dipped in molten silver, thus applying a silver coating to the insert. Silver is then melted in the holder recess, whereupon the insert is inserted into the recess and the holder and insert are heated until silver flows around the insert.
However, both Severance and Weatherly suggest the use of a sleeve or coating which requires a substantial amount of silver to fabricate. Given the cost of silver, and the fact that erosion of the electrode will necessarily require replacement electrodes to be purchased, there is still a major cost consideration for the user of this plasma torch technology.